With the progress of computer image processing techniques, a method of directly writing images on a photosensitive layer by light irradiation in response to digital signals has recently been developed and, thus, intense interest has been shown toward a computer-to-plate (CTP) system in which images are directly formed on a photosensitive lithographic printing plate, without outputting them onto a silver salt mask film, by employing the method in a lithographic printing plate precursor. The CTP system, which uses a high-output laser having a maximum intensity within the near infrared or the infrared range as a light source for light irradiation, has the following advantages: images having high resolution can be obtained by exposure within a short time and the photosensitive lithographic printing plate used in the system can be handled in a daylight room. Regarding solid and semiconductor lasers capable of emitting infrared ray having a wavelength of 760 to 1200 nm, high-output and portable lasers readily available.
As a positive type photosensitive lithographic printing plate material for a CTP system, for example, a printing plate material obtained by adding a photothermal conversion material and a quinonediazide compound to an alkali soluble resin is known. In the image area of the positive type lithographic printing plate, the quinonediazide compound functions as a dissolution inhibitor which substantially decreases the alkali solubility of the alkali soluble resin. In the non-image area, the quinonediazide compound is decomposed by heat to lose the dissolution inhibiting capability, and thus the alkali soluble resin is removed by an alkali developer to form an image.
As a negative type photosensitive lithographic printing plate for CTP system, there is known a printing plate in which, by introducing a substance which generates an acid due to light or heat into a photosensitive layer, the condensation crosslinking reaction is caused by a heat treatment after exposure using the acid generated on exposure as a catalyst, and the photosensitive layer of the exposed area is cured to form an image. There is also known a printing plate in which, by introducing a substance which generates a radical due to light or heat into a photosensitive layer, a polymerization reaction is caused using the acid generated on exposure as a catalyst, and the photosensitive layer of the exposed portion is cured to form an image.
Regarding the support used in such a lithographic printing plate precursor, intensive research has hitherto been done into making the surface of the support hydrophilic so as to prevent stains in the non-image area. For example, when a metal support such as aluminum plate is used as a substrate, various techniques such as anodizing of aluminum substrate, or silicate treatment of an anodized aluminum substrate for further improving hydrophilicity have been proposed.
However, various treatments for improving hydrophilicity are not always excellent in affinity to the photosensitive layer. There has also been a problem that, in some cases, adhesion between the support and the photosensitive layer formed thereon is impaired so that the photosensitive layer is peeled under severe printing conditions and sufficient printing durability cannot be obtained.
Thus, there was suggested a method in which various subbing layers are formed between the support and the photosensitive layer so as to improve adhesion between the hydrophilized support surface and the photosensitive layer. When a material having a functional group which is excellent in affinity to a resin material constituting the photosensitive layer or the support surface is used as the subbing layer, adhesion in the image area is improved and a sufficient printing durability can be obtained.
However, in the non-image area, the photosensitive layer is not completely removed on development, and remains the layer on the support surface, thereby to cause a problem that adhesion of ink onto the remaining layer causes stains in the non-image area. Therefore, it has been desired to develop a lithographic printing plate precursor which has excellent surface hydrophilicity and also satisfies both excellent adhesion to a photosensitive layer in the image area and excellent removability of the photosensitive layer in the non-image area.
As the lithographic printing plate precursor, there is proposed a lithographic printing plate precursor provided with an intermediate layer comprising a polymer containing a monomer having an acid group and a monomer having an onium group (see Patent Document 1). This lithographic printing plate precursor improves adhesion of a support and a photosensitive layer, thereby to further improve printing durability and to suppress generation of stains in the non-image area. However, it has been desired to further decrease the generation of stains in the non-image area and to improve development latitude.
[Patent Document 1] Japanese Unexamined Patent Publication (Kokai) No. 2000-108538